Transgender is when someone feels like they are the opposite sex assigned at birth.
When did I know I was transgender? From birth til I was 12 I lived as a boy. When I hit puberty I immediately knew I didn't like girls so I threw that out the window & identified as a gay male. Although from the point I knew I liked boys, I considered myself a girl. This was because I wasn't into gay males & I knew I didn't act like a typical boy or fit in with typical boys so I just considered myself a girl & when people would ask me if I was gay I would just respond, "I'm a girl but in a boys body". Remember, this is when I was 12! I had no clue someone could even become a girl. It's just what I saw myself as. Then when I got to about 13 I learned about other kids who felt like girls & their families accepted them & they were living as the girl they felt they were. So at that point, I was already dressing girly, painting my nails, wearing purses, etc. & just told myself I'd begin living as a girl & searching for hormones. I would say I officially began transitioning in public when I was 14 but knew I was trans when I was 13.